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Atmospheric chemistry II
Oxidizing atmosphere
• The main oxidants in the atmosphere are
– OH radical (·OH)
– Ozone
– Nitrate Radical (·NO3)
The OH radical: main tropospheric oxidant
O3 + hvO2 + O(1D) (1)
O(1D) + M O + M (2)
O(1D) + H2O 2OH (3)
Primary source:
Sink: oxidation of reduced species
CO + OH CO2 + H
CH4 + OH CH3 + H2O
HCFC + OH H2O +…
Major
OH sinks
GLOBAL MEAN [OH] = 1.2x106 molecules cm-3
and a lifetime of about 100 ms
Earths surface, 30º N
D.J. Jacob
Tropospheric OH production takes place
in a narrow UV window (300-320 nm)
30 equinox
midday
Solar spectrum
D.J. Jacob
O(1D)
RO2
HONO HCHO
OH HO2 H2O2 + O2HNO3
H2SO4 RO2 + O2
RO2
NOhν
NO2
SO2
NO O3
CH4 HCHO H2
CO NMHC O3
HO2
hν
VOC
H2O
(Boy et al., ACP, 2005)
Formation of HO2 Radical, examples
HCHO + hv ·H + HCO
·H+O2 +M·HO2+M
HCHO + hv H2 + CO
CO + ·OH CO2 + ·H
·H+O2 +M·HO2+M
Reaction of HO2 To be included
• OH + O3 = HO2 : 1.70D-12*EXP(-940/TEMP) ;
• OH + H2 = HO2 : 7.70D-12*EXP(-2100/TEMP) ;
• OH + CO = HO2 : 1.30D-13;
• OH + H2O2 = HO2 : 2.90D-12*EXP(-160/TEMP) ;
• HO2NO2 = HO2 + NO2 : 5.2D-6*EXP(19900/TEMP) ;
• NO3 + H2O2 = HNO3 + HO2 : 4.1D-16 ;
• HCHO = CO + HO2 + HO2 : J(9);
– Cross section:3.60 D-20 cm2
– Quantum yield:0.9
Reaction of HO2 To be included
• HO2 + O3 = OH : 2.03D-16*((TEMP/300)**4.57)* EXP(693/TEMP) ;
• OH + HO2 = dummy : 4.80D-11*EXP(250/TEMP) ;
• HO2 + HO2 = H2O2 : 2.20D-13*EXP(600/TEMP);
• HO2 + NO = OH + NO2 : 3.60D-12*EXP(270/TEMP) ;
• HO2 + NO2 = HO2NO2 : 1.4D-13 ;
• HO2 + NO3 = OH + NO2 : 4.00D-12 ;
Reactions of OH to be included
• All HO2 including OH
• O1D = OH + OH : 2.20D-10*H2O ;
• H2O2 = OH + OH : J(3) ;– Cross section:33.04D-20 cm2
– Quantum yield:0.49
• HONO = OH + NO : J(7);– Cross section:5.19D-19 cm2
– Quantum yield:0.40
• HNO3 = OH + NO2 : J(8);– Cross section:4.29D-18 cm2
– Quantum yield:0.88
Reactions of OH to be included
• OH + NO = HONO : 1.50D-11*EXP(TEMP/300)**-0.5 ;
• OH + NO2 = HNO3 : 2.4S-11*(TEMP/300)**-1.7 ;
• OH + NO3 = HO2 + NO2 : 2.00D-11 ;
• OH + HO2NO2 = NO2 : 1.90D-12*EXP(270/TEMP) ;
• OH + HONO = NO2 : 2.50D-12*EXP(-260/TEMP) ;
• OH + HNO3 = NO3 : 7.20D-15*EXP(785/TEMP);
Formation of Sulfuric acid
• SO2 + ·OH + M ·HOSO2 + M
• ·HOSO2 + O2 ·HO2 + SO3
• H2O + SO3+ M H2SO4 + M
Reactions to be included
1. O + SO2 = SO3 : 4.00D-32*EXP(-1000/TEMP)*M ;
2. OH + SO2 = HSO3 : 1.5D-12*(TEMP/300) ;
3. HSO3 = HO2 + SO3 : 1.30D-12*EXP(-330/TEMP)*O2 ;
4. SO3 = H2SO4 : 5.D-15*H2O ;
How d[SO3]/dt should look like?
= K1[O][SO2] + k3[HSO3]-k4[SO3]
Monoterpenes oxidation by OH radicals
Reaction included in the model
• APINENE + OH = APINAO2 :1.20D-11*EXP(444/TEMP) ;
Ozone in the troposphere
Greenhouse gas
Toxic pollutant in surface air
But it is the main precursor of OH and plays therefore
a key role in maintaining the oxidizing power of
the troposphere
O3 + hvO2 + O(1D) (1)
O(1D) + M O + M (2)
O(1D) + H2O 2OH (3)
Tropospheric ozone (O3): Global budget
O3
O2h
O3
OH HO2
h, H2O
Deposition
NO
H2O2
CO, VOC
NO2
h
STRATOSPHERE
TROPOSPHERE
8-18 km
1165Deposition475Transport from
stratosphere
4230Chem loss in
troposphere4920Chem prod in
troposphere
GEOS-CHEM model budget terms, Tg O3 yr-1
D.J. Jacob
H2O
Depending on the NO/HOx ratio
Ozone production:
CO + ·OH → ·H + CO2
·H + O2 + M → ·HO2 + M
(b) HO2 + NO → OH + NO2
NO2 + hν (λ < 420 nm) → NO + O
O + O2 → O3
Ozone destruction:
CO + OH → H + CO2
H + O2 + M → HO2 + M
(a) HO2 + O3 → 2O2 + OH
Ra/Rb = Ka[HO2][O3] / Kb[HO2][NO] =2.5E-4 * [O3]/[NO]
Break-even concentration ≈ 2.5 10-4 (20 ppb O3 ≈ 5 ppt NO)
Reactions to be included in the model
• O = O3 : 5.60D-34*O2*N2*((TEMP/300)**-2.6) + 6.00D-34*O2*O2*((TEMP/300)**-2.6) ;
• O3 = O1D : J(1) ; – Cross section:1.137D-17 cm2
– Quantum yield:0.8
• O + O3 = dummy : 8.00D-12*EXP(-2060/TEMP) ;
• NO + O3 = NO2 : 1.40D-12*EXP(-1310/TEMP) ;
• NO2 + O3 = NO3 : 1.40D-13*EXP(-2470/TEMP) ;
• And all those involving O3 shown in OH and HO2 sections
Monoterpenes Oxidation by Ozone
• In a first step, ozone will be added to the double bond forming a cyclic structure:
D-Limonene oxidation by Ozone
APINENE + O3 = 0.77 * OH + APINOOA : 1.01D-15*EXP(-732/TEMP) ;
The nitrate radical – NO3
The third important oxidant is the nitrate radical
NO2 + O3 NO3→ + O2
and is in equilibrium with N2O5 according to
NO2 + NO3 N2O5↔M+ + M
During daytime it photolyze rapidly by two reactions
NO3 + hν (λ700>nm)→NO+O2
NO3 + hν (λ580>nm)→NO2 + O
Typically NO3 mixing ratios:
daytime≈fewppts
night≈uptoseveralhundredsofppt
Reactions to be include• All above and:
• NO2 + O3 = NO3 : 1.40D-13*EXP(-2470/TEMP) ;
• NO + NO3 = NO2 + NO2 : 1.80D-11*EXP(110/TEMP) ;
• NO2 + NO3 = NO + NO2 : 4.50D-14*EXP(-1260/TEMP) ;
• NO2 + NO3 = N2O5 : 1.50D-12*(TEMP/300)**-0.7) ;
• N2O5 = NO2 + NO3 : 9.7D14*((TEMP/300)**-0.1)*EXP(-11080/TEMP) ;
• NO3 + H2O2 = HNO3 + HO2 : 4.1D-16 ;
• NO3 + NO3 = NO2 + NO2 : 8.5D-13*EXP(-2450./TEMP) ;
Reactions to be include
• NO3 = NO : J(5);
– Cross section: 4.58D-17 cm2
– Quantum yield:0.35
• NO3 = NO2 + O : J(6) ;
– Cross section:4.58D-17 cm2
– Quantum yield:0.60
Monoterpenes NO3 oxidation
APINENE + NO3 = NAPINAO2 : 1.19D-12*EXP(490/TEMP) ;
A word on organics
• Note that in our model we only will use alpha-pinene, and only 1 oxidation path per oxidant.
• Also we will use a fixed concentration of alpha pinene. In next lectures you will learn how to implement emissions in your model.
Starting the methane oxidation is the reaction with OH:
CH4 CH3→OH+ + H2O
The methyl radical reacts rapidly adds O2:
CH3 + O2 CH3O2→
The methylperoxy radical is considered part of the HOx-family and its main sinks are the reaction with HO2 and NO.
In the overall following reaction chain the C(-IV) atom in CH4 is successively oxidized to C(+IV) in CO2.
Consider high NOx-atmosphere:
CH4 + 10 O2 -> CO2 + H2O + 5 O3 + 2 OH
Consider now a low NOx-situation:
CH4 + 3 OH + 2 O2 -> CO2 + 3 H2O + HO2
Ozone production and radical conversion or source is only reached in a high NOx-atmosphere. This results show the critical role of NOx for maintaining O3 and OH in the troposphere.
Methane oxidation mechanism
NO NO2
Oxidation of methane
CH4 + OH kCH4+OH = 9.65x10-20∙T
∙exp(-1082/T) cm3 Molek.-
CH3 + H2O (+O2)
CH3O2
HCHO
CH3OOH
+HO2+RO2
0.67 HCHO + CH3OH+NO2
CH3OONO2
kCH3O2+HO2= 3.80x10-13∙exp(780/T) cm3 Molek.-1 s-1
kCH3O2+RO2= 1.82x10-13∙exp(416/T) cm3 Molek.-1 s-1
kCH3O2+NO = 3.00x10-12∙exp(280/T) cm3 Molek.-1 s-1
